If you're OK with not mentioning poison chance, this should fit.
"Picks most damaging of Phys+Contact or Spc."
If you want to mention poison chance and contact, this is the best I could come up with.
"20% psn. Phys+Contact if that damages more."
If you want to mention poison, physical, and special attack possibility,
"20% psn. Picks stronger of Phys+Contact, Spc."
If you don't want to use "psn",
"20% Poison. Phys+Contact if would be stronger.